A serial transport frame format method was previously disclosed in U.S. Pat. No. 5,060,229 to Tyrrell et al. That format method involves interfacing a high speed parallel bus and a low speed serial bus in a network element that receives and sends high speed signals from and to similar network elements and that further switches low speed signals to and from the network. The switching matrix in the network element is responsive to high speed signals from two different directions and passes on some of those high speed signals to other network elements. It also switches others of the high speed signals down to local subscribers at a lower speed. Similarly, local subscribers provide low speed serial multiplexed DS-1 signals to the network element which in turn switches some or all of such signals on to the high speed busses for transmission to other network elements. In the network element disclosed in the above cited U.S. Patent, an internal serial transport frame format is described which is used to transfer data between the switching matrix and a plurality of serial bus interface modules. Each of these provide a number of DS-1 signals to a corresponding number of asynchronous modules for further decomposition or construction, respectively, into or from even lower speed DS-0 signals, corresponding to individual subscribers, each operating serially at 64 Kb/s. The DS-1 interfaces can incorporate up to 24 DS-0 channels and operate at the electrical telephony standard of 1.544 Mb/s. The signals between the serial bus interfaces and the switching matrix operate at about 8.192 MHz. The signals used to interface between network elements are operating at even higher speeds, e.g., 51.84 Mb/s (STS-1) and higher, according to ANSI standard T1.105 entitled "Digital Hierarchy Optical Interface Rates and Formats Specification". An STS-1 frame is shown in FIG. 1. The frame repetition rate is 8 kHz. The frame contains one byte of voice data from a large number of subscribers depending on how many bytes in the STS-1 SPE are used for non-voice purposes. For example, in the U.S., there are 672 bytes used of the 756 available in the SPE.
The serial transport frame format disclosed in the above mentioned U.S. patent defines a frame for the transfer of 32 channels of information between the switch matrix and each of the 28 DS-1 links on the other side of the serial bus interfaces. Each channel comprises 16 bits (8 data bits) and each frame is generated at a selected frequency of, e.g., 4.096 MHz. As may be seen in FIG. 2, a first portion of each channel contains the voice data while a second portion of the same channel contains associated control information., Each channel comprises 16 bits represented on 16 wires between the switching matrix and the serial bus interface. The control information may include timing information associated with that channel of data.
When the channels contain DS-0 data, the first 8 bits of each 16 bit channel contains the actual DS-0 data (such as voice data) with the remaining bits containing the signalling information (e.g., A, B or A, B, C, D information) and timing information associated with that DS-0, channel as well as channel parity information.
To facilitate the transport of lower rate digital signals, the SONET standard uses sub-STS-1 payload mappings, referred to as Virtual Tributary (VT) structures. There are currently 4 sizes of VT as shown in FIG. 3. This mapping divides the SPE frame of FIG. 1 into 7 equal sized subframes or VT blocks with 12 columns (108 bytes) in each. Thus, the subframes account for (7) (12)=84 columns with the path overhead column and 2 unused columns (reserved bytes R) accounting for the remainder of the 87 columns in an SPE. The rate of each VT structure is determined as (108) (8) (8,000)=6.912 megabits/s.
The VT structure described above is designed to facilitate consistent transport and switching of various payloads uniformly by handling only VTs. All services below the DS-3 rate are transported within a VT structure.
A VT group of 12 columns can be individually assigned to carry one of the four types of signals shown in FIG. 3. Depending on the data rate of the particular VT type, more than one signal may be carried in a 12 column VT structure as a VT group. Thus, the VT1.5 type group can carry four VT1.5s in a 12 column VT group. Similarly, three VT2.0s can be carried in a VT2 type group. Only two VT3.0s and only one VT6.0 can be carried in a group. When all seven VT groups in an envelope are VT1.5s, a total capacity of 28 DS-1's is provided-the same as a DS-3 signal. This approach is attractive in that the virtual tributaries are individually accessible for cross connect or add/drop multiplexer systems.
For VT1.5 groups, the serial transport frame format shown in FIG. 2 is conveniently designed to accommodate 24 DS-0's in channels 4-27 while the remaining channels are used for internal management purposes or are not assigned. One such frame thus accommodates one DS-1 signal and is called a link. There may be seven such links between each of, e.g., four serial bus interfaces and the switching matrix. Altogether, there will thus be 28 DS-1 links between the switching matrix and the serial bus interfaces. Additional links may also be added for control purposes, e.g., "VI".
The serial transport frame format of the above mentioned U.S. patent is particularly well suited to be used for transporting the VT1.5 size virtual tributary. There was a possibility, however, that we could adapt hardware designed for that format to other virtual tributary sizes, such as VT2, which might be desirable for application in countries in which that format is used.